This invention relates to limited-rotation motors or galvanometers, and more particularly to that class in which the rotor is supported on ball bearings. The invention also relates to a novel bearing unit that uses a preload characterized by a high spring constant.
These devices often perform precision tasks that require a high degree of accuracy. Common uses of these devices include electronic manufacturing and repair operations in which a laser beam is directed to perform tasks such as the profiling, marking, cutting, drilling, and trimming of silicon and other semi-conducting materials. The galvanometers are also used in high-precision optical scanning operations in which a laser beam is swept over a field of interest and reflections of the beam are sensed and analyzed. To perform these precision tasks, limited-rotation motors are equipped with means for monitoring and reporting the instantaneous angular positions of the rotors.
As described herein, the rotor may comprise a cylindrical permanent magnet armature and a pair of shafts that support the armature for rotation in ball bearings. Several factors influence the service life of these bearings and the axial position of the rotor, which in turn affect the accuracy of these devices. For example, in high precision applications of the galvanometer, bearing slackness, which results in radial movements of the rotor, results in substantial degradation of the accuracy of the beam positioning. Therefore, the bearings are preloaded by means of a spring arrangement to remove the slack. However, conventional preloading systems do not perform satisfactorily in the applications to which this invention is directed.
More specifically, a substantial amount of heat is generated in a galvanometer in a continuous-use environment such as high-speed scanning. The resulting temperature changes cause expansion and contraction of the armature. If the preload spring has a high spring constant, these dimensional changes can cause a relatively large change in the force exerted by the preload spring, resulting in excessive bearing wear in one direction and inadequate preload in the other direction.
A preload spring having a relatively low spring constant will avoid excessive changes in the preload force in response to thermal expansion and contraction. However, the low spring constant causes another problem. Because of various asymmetries in the system, a time-varying axial force is exerted on the rotor. The resulting axial displacement of the rotor is a chatter characterized by a resonance involving the mass of the rotor (and load) and the spring constant of the preload spring. If the resonant frequency is within the passband of the servo system that drives the galvanometer, the relatively large axial displacement at this frequency will be sensed by the angular-position sensor that provides a feedback signal. This will cause instability in the servo system. Accordingly, the passband is limited to frequencies below the resonance, which has a low frequency owing to the low spring constant.
In addition, conventional galvanometer design has not recognized that axial placement of the bearings, stator drive coils, and stator back iron can also profoundly affect the axial forces applied to the bearings during acceleration of the rotor assembly. As a result, insufficient attention has been devoted to placing these elements in an axially symmetric relationship with respect to the magnetic center of the armature, and unnecessarily large axial forces have been applied to the bearings, shortening their life considerably.
Therefore, what is needed, is an efficient, high-performance galvanometer bearing assembly design, which extends the service life of the bearings, while providing accurate performance. Such a bearing design would provide the stiffness of a high preload design, thereby avoiding problems resulting from chatter and would also provide the compliance of a low preload design that would account for bearing wear and differential thermal expansion of the rotor components.